Air is separated into its component parts by distillation that is conducted in air separation plants. Such plants employ a main air compressor to compress the air, a prepurfication unit to remove higher boiling contaminants from the air, such as carbon dioxide, water vapor and hydrocarbons, and a main heat exchanger to cool the resulting compressed and purified air to a cryogenic temperature suitable for its distillation within a distillation column unit. The distillation column unit employs a higher pressure column, a lower pressure column and optionally an argon column when argon is a desired product.
The compressed air is introduced into the higher pressure column and is rectified into a crude liquid oxygen column bottoms, also known as kettle liquid, and a nitrogen-rich vapor column overhead. A stream of the crude liquid oxygen is introduced into the lower pressure column for further refinement into an oxygen-rich liquid column bottoms and a nitrogen-rich vapor column overhead. The lower pressure column operates at a lower pressure to enable the oxygen-rich liquid to condense at least part of the nitrogen-rich vapor column overhead of the higher pressure column for purposes of refluxing both columns and for production of nitrogen products from the condensate. Streams of the oxygen-rich liquid, nitrogen-rich vapor and condensed nitrogen-rich vapor can be introduced into the main heat exchanger to help cool the air and warmed to produce oxygen and nitrogen products.
Where argon is a desired product, an argon column can be connected to the lower pressure column to rectify a stream of an argon and oxygen containing vapor removed from the lower pressure column. Furthermore, when an oxygen and/or a nitrogen product is desired at high pressure, potentially a supercritical pressure, a stream of the oxygen-rich liquid produced as column bottoms in the lower pressure column and/or a stream of nitrogen-rich liquid produced as condensate can be pumped and then heated in a heat exchanger to produce a high pressure vapor or a supercritical fluid. Typically, the heat exchange duty for such purposes is provided by further compressing part of the air in a booster compressor after the air has been compressed in the main air compressor. The resulting boosted pressure air stream is liquefied and the liquid air stream can be introduced into either the higher pressure column or the lower pressure column or both of such columns.
As can be appreciated, the degree to which oxygen is present within the column overhead of the lower pressure column depends primarily upon the reflux ratio within the upper sections of lower pressure column. As reflux ratio (L/V) is increased a greater proportion of the oxygen and argon will be extracted from the lower pressure column at a lower level (eventually recovered as product oxygen or argon). Typically, in plants employing a pump to pressurize a product with resulting liquefied air, at least a portion of the liquid air is introduced into the lower pressure column above the location or locations at which the crude liquid oxygen is introduced. This introduction of liquid air increases the liquid to vapor ratio below the point of introduction to that L/V which would have existed relative to the top of the column or that which would have existed if the liquid air was not fed to the upper column. This decreases the amount of oxygen within the column overhead of the lower pressure column and in turn increases oxygen recovery.
As will be discussed, the present invention provides a method and apparatus for separating air in which a subcooled liquid is produced that has both an oxygen and a nitrogen content and argon content that is no less than air and such subcooled liquid is introduced into the lower pressure column above a region thereof at which the crude liquid oxygen is introduced to decrease the degree to which oxygen is present within the overhead of the lower pressure column to an extent that is greater than conventionally obtained by the introduction of liquid air as in the prior art.